The present invention relates to AGC amplifiers and particularly such amplifiers for predetection combiners.
In a predetection combiner of the type used in a receiver for a tropospheric scattering communications system, component signals which are received on a plurality of antennas are adjusted to have equal phase and combined to provide a composite signal with enhanced signal-to-noise ratio. The receiver circuits prior to the combiner usually include automatic gain control (AGC) amplifiers in each channel, which adjust the gain of each channel to provide a combined composite signal with a desired power level.
In such a combining circuit it is important to provide accurate tracking of the gain of each amplifier, so that all channels will have the same gain in response to a specified level of AGC control signal. Gain control is usually provided by PIN-diode attenuators in each channel.
In order to attain near theoretical combining results, several factors must be considered. The most important factors include equal front end noise and coherent modulation in each component signal channel, and equal and identical gain characteristics of each component channel. Of the aforementioned factors, only the gain characteristic is within the control of the combiner designer.
To accomplish equal gain transfer characteristics in each of two or more channels, prior systems resort to complex circuitry and tedious alignment procedure. The circuits provide predistortion of AGC voltages. Circuit components are selected for matched characteristics, and hours of alignment are required to achieve tracking errors in the order of 0.5 to 1 dB over the dynamic range of 60 dB. It is well known that the attenuation provided by a PIN-diode attenuator is a non-linear function of the voltage applied to the diode. Attempts in the prior art to provide a linear characteristic for such attenuators have generally taken the approach of providing a non-linear analog circuit, which compensates for the non-linear characteristics of the diode. A recent article showing this approach was published by Gary M. Hover in Microwaves & RF, April, 1983 pp. 83-84, entitled "PIN-Diode Driver Allows Linear RF Attenuation". This approach is not capable of yielding high accuracy for attenuation as a function of applied AGC voltage. Another problem with this approach is the fact that diode characteristics can vary from one part to another, and this variation can provide substantial inaccuracies in the resulting product unless the analog circuit is individually adjusted for each diode.
It is therefore an object of the present invention to provide a predetection combiner having AGC amplifiers with improved linearity.
It is a further object of the present invention to provide an AGC amplifier with linear control characteristics.
It is a still further object of the present invention to provide an AGC amplifier with digital control.
It is a still further object of the present invention to provide a linear attenuator with digital control.